Aromatic polyester resins, as represented by polyethylene terephthalate, are excellent in shapability, mechanical properties, transparency, etc. and are widely used as a packaging material for various foods and containers for beverages, etc. However, as a packaging material, particularly for foods to be stored for a long period, the gas-barrier property of an aromatic polyester resin is not sufficient so that the deterioration of contents has been inevitable.
On the other hand, polyglycolic acid resin is known to have particularly excellent gas-barrier property in addition to heat resistance and mechanical strength (e.g., Patent document 1 listed below), and it has been proposed to add a small amount thereof to an aromatic polyester resin to provide an aromatic polyester resin composition improved in gas-barrier property of the latter (Patent documents 2 and 3). However, an aromatic polyester resin having ordinarily a melting point of at least 240° C. and a polyglycolic acid resin having a melting point of ca. 200° C. do not necessarily have good mutual solubility, and for obtaining a uniform mixture of these resins, it is necessary to effect melt-kneading at a temperature exceeding the melting points of both resins. During such melt-kneading and the melt-forming of the resultant mixture composition, a considerable amount of gas generation was observed, and there have arisen serious problems in commercial production of an aromatic polyester resin/polyglycolic acid resin mixture composition, such as deterioration of environments for melt-processing operations including such melt-kneading and melt-forming, and soiling of the processing apparatus and the product after the processing with the condensed and attached gas components.    Patent document 1: JP-A 10-60136    Patent document 2: U.S. Pat. No. 4,565,851    Patent document 3: JP-A 2005-200516.